The pyroelectric infrared detection element (a pyroelectric element) detects infrared by use of the pyroelectric effect. Such a pyroelectric infrared detection element has been frequently used for an infrared sensor such as a human body detection sensor for detecting a movement of a human body.
The pyroelectric effect is a phenomenon that electric charges are generated on a surface in response to a change in a temperature. When infrared strikes the pyroelectric infrared detection element in an equilibrium state in which electric charges resulting from spontaneous polarization are neutralized by ions from an external air, the infrared heats a pyroelectric substrate, and a temperature change of the pyroelectric substrate occurs. Such a temperature change may break the equilibrium state of electric charges, and then electric charges are generated on the surface of the pyroelectric substrate.
As an infrared sensor using a pyroelectric infrared detection element, an infrared sensor that a pyroelectric infrared detection element and a current voltage conversion circuit are housed in a single package has been well known. The current voltage conversion circuit is configured to convert a current caused by a movement of electric charges generated at the pyroelectric infrared detection element into a voltage signal and then outputs the resultant voltage signal. In such an infrared sensor, the pyroelectric infrared detection element has impedance of about 100 GΩ, and a current (output current) outputted from the pyroelectric infrared detection element is very weak. In view of the above, a current voltage conversion circuit employing an electric field effect transistor (FET) and a resistor has been well used. In such a current voltage conversion circuit, the electric field effect transistor is used for impedance conversion and has its gate connected to the pyroelectric infrared detection element, and the resistor is used for determining a gate voltage of the electric field effect transistor.
As the above pyroelectric infrared detection element, a dual element (a dual-type pyroelectric infrared detection element) and a quad element (a quad-type pyroelectric infrared detection element) have been put to practical use. The dual element is defined as an element in which two light receiving parts (infrared receiving parts) are provided to a single pyroelectric substrate. The quad element is defined as an element in which four light receiving parts are provided to a single pyroelectric substrate. Besides, as the above pyroelectric infrared detection element, a single element (a single-type pyroelectric infrared detection element) also has been put to practical use. The single element is defined as an element in which a single light receiving part is provided to a single pyroelectric substrate.
In the above pyroelectric infrared detection element, the pyroelectric substrate is made of pyroelectric material, such as, ceramic material (e.g., PbTiO3, PZT, PZT-PMN (: Pb(Zr,Ti)O3—Pb(Mn,Nb)O3), single-crystal material (e.g., LiTaO3), and high-polymer material (e.g., PVF2). In addition, the light receiving part is constituted by paired two electrodes formed on opposite surfaces of the pyroelectric substrate in a thickness direction so as to be opposite to each other and a part of the pyroelectric substrate interposed between the paired two electrodes. Besides, each electrode may be made of electrically conductive infrared absorption material (e.g., NiCr).
Further, as the above pyroelectric infrared detection element, an pyroelectric infrared detection element in which a plurality of the light receiving parts is formed at a center of the pyroelectric substrate and output terminal units are formed at respective opposite ends of the pyroelectric substrate and wiring parts are formed to connect the electrodes of each light receiving part to the output terminal units respectively has been well known (c.f., document 1: JP 3773623 B2). In the pyroelectric infrared detection element disclosed in document 1, the electrodes of the light receiving part, the output terminal units, and the wiring parts are made of NiCr.
With regard to the infrared sensor using the pyroelectric infrared detection element, even when the light receiving part of the pyroelectric infrared detection element receives no infrared from a detection object (e.g., a human body), a false operation is likely to occur due to a temperature change of surrounding environment (usage environment). In view of the above, in such an infrared sensor, a dual-type pyroelectric infrared detection element which is configured such that external noise occurring simultaneously in the respective two light receiving parts cancels each other out is used (e.g., document 2: WO 2006/120863 A1, and document 3: WO 2006/112122 A1).
Document 3 discloses an infrared sensor that a pyroelectric infrared detection element is accommodated in a package. The package is constituted by a package body and an optical filter. The package is shaped into a box having an opened surface. The optical filter is configured to transmit infrared and is placed so as to cover the opened surface of the package body. In addition, document 3 discloses that the package body made of electrical insulation ceramic can be used as an alternative of the package body made of metal.
Further, in the past, there has been proposed a pyroelectric infrared detection element capable of suppressing popcorn noise occurring in an unexpected fashion due to a temperature change of surrounding environment (c.f., document 4: JP 10-300570 A). In the pyroelectric infrared detection element disclosed in document 4, a single-crystal LiTaO3 substrate has a first part interposed between paired two electrodes and a second part other than the first part. The first part is designed to have a single domain structure with a defined spontaneous polarization direction. The second part is designed to have a multi-domain structure with a randomly oriented spontaneous polarization direction.
In the past infrared sensor, for example, it is considered that a path in which heat resulting from a temperature change in the surrounding environment is transferred to the pyroelectric infrared detection element may include a path in which the resultant heat is transferred from the package to the pyroelectric infrared detection element via gas inside the package, a path in which the resultant heat is transferred from the package to the pyroelectric infrared detection element via object supporting the pyroelectric infrared detection element inside the package, and a path in which the resultant heat is transferred from the package to the pyroelectric infrared detection element via heat emission.
Consequently, with regard to the prior infrared sensors, it is considered that influences given to the light receiving part by the temperature change in the surrounding environment may be different depending on a shape or material of the package, a distance between the package and the pyroelectric infrared detection element, and position relations among circuit components constituting the current voltage conversion circuit and the pyroelectric infrared detection element.
Hence, merely using a dual-type or quad-type pyroelectric infrared detection element as the pyroelectric infrared detection unit is not enough to cancel influences caused by temperature changes other than temperature changes simultaneously occurring in the respective light receiving parts with regard to external noise. In brief, the prior infrared sensor can cancel influences caused by a temperature change in a specified direction of the pyroelectric infrared detection element, but is likely to be affected by influences caused by a temperature change in a direction other than the specified direction.
With regard to the pyroelectric infrared detection element, when the temperature of the pyroelectric substrate is changed, electric charges are generated at entire opposite surfaces of the pyroelectric substrate in the thickness direction. Hence, in the prior pyroelectric infrared detection element, electric charges generated at a site other than the light receiving parts in addition to electric charges generated at the light receiving parts are outputted via the output terminal unit to the current voltage conversion circuit provided as an external circuit. Thus, the S/N ratio is likely to be decreased due to a change in the temperature of the surrounding environment.
Further, document 4 discloses a process of fabricating the pyroelectric infrared detection element. In this process, first, electrodes are formed on opposite surfaces in a thickness direction of a single-crystal LiTaO3 substrate. Thereafter, the LiTaO3 substrate is heated up to the Curie temperature so as not to show pyroelectric properties. Subsequently, the LiTaO3 substrate is cooled down to a room temperature while a high electric field is applied between the opposite electrodes. Consequently, adopting the process disclosed in document 4 increases the number of steps of the process for fabricating the pyroelectric infrared detection element, and therefore the production cost thereof may be increased.